test_approx has ambiguities

[mcabbott]

julia> ChainRulesTestUtils.test_approx(Tangent{Int}(; x=1), ZeroTangent())
ERROR: MethodError: test_approx(::Tangent{Int64, NamedTuple{(:x,), Tuple{Int64}}}, ::ZeroTangent) is ambiguous. Candidates:
  test_approx(x, ::AbstractZero) in ChainRulesTestUtils at /Users/me/.julia/packages/ChainRulesTestUtils/8dFTY/src/check_result.jl:34
  test_approx(actual::Tangent{P, T}, expected) where {T, P} in ChainRulesTestUtils at /Users/me/.julia/packages/ChainRulesTestUtils/8dFTY/src/check_result.jl:108
Possible fix, define
  test_approx(::Tangent{P, T}, ::AbstractZero) where {T, P}
Stacktrace:

That's a made-up example, the real one gave:

Error During Test at /Users/me/.julia/packages/ChainRulesTestUtils/8dFTY/src/testers.jl:195
  Got exception outside of a @test
  MethodError: (::ChainRulesTestUtils.var"#test_approx##kw")(::NamedTuple{(:rtol, :atol), Tuple{Float64, Float64}}, ::typeof(test_approx), ::ChainRulesCore.NoTangent, ::ChainRulesCore.Tangent{Tuple{Float64, Float64, Float64}, Tuple{Float64, Float64, Float64}}) is ambiguous. Candidates:
    (var"#s103"::ChainRulesTestUtils.var"#test_approx##kw")(::Any, ::typeof(test_approx), x, y::ChainRulesCore.Tangent) in ChainRulesTestUtils at /Users/me/.julia/packages/ChainRulesTestUtils/8dFTY/src/check_result.jl:124
    (var"#s102"::ChainRulesTestUtils.var"#test_approx##kw")(::Any, ::typeof(test_approx), ::ChainRulesCore.AbstractZero, x) in ChainRulesTestUtils at /Users/me/.julia/packages/ChainRulesTestUtils/8dFTY/src/check_result.jl:33
  Possible fix, define
    (::ChainRulesTestUtils.var"#test_approx##kw")(::Any, ::typeof(test_approx), ::ChainRulesCore.AbstractZero, ::ChainRulesCore.Tangent)

Real example: https://github.com/JuliaDiff/ChainRules.jl/pull/570/files#diff-14d216cb53721d9360bbbcc399d93fff67f2594bd388162abbbe82c59ad1ba9fR169